1. The Field of the Invention
The present invention is related generally to optical signal transmission. More specifically, the present invention relates control of optical transmission parameters.
2. Related Technology
High-speed communication networks have become of increased importance as the need to transmit large amounts of data continues to rise. Networks employing fiber optic technology are known as optical communications networks, and are typically characterized by high bandwidth and reliable, high-speed data transmission. Fiber optic technology is increasingly employed in the transmission of data over high-speed communications networks.
To communicate over a network using fiber optic technology, fiber optic components such as fiber optic transceivers are used to send and receive optical data. Generally, a fiber optic transceiver can include one or more optical subassemblies (“OSA”) such as a transmitter optical subassembly (“TOSA”) for sending optical signals, and a receiver optical subassembly (“ROSA”) for receiving optical signals. More particularly, the TOSA receives an electrical data signal and converts the electrical data signal into an optical data signal for transmission onto an optical network. The ROSA receives an optical data signal from the optical network and converts the received optical data signal to an electrical data signal for further use and/or processing. Both the ROSA and the TOSA include specific optical components for performing such functions.
In particular, a typical TOSA can include an optical transmitter, such as a laser, for sending an optical signal. Many different types of lasers are known to those skilled in the art. One type of laser referred to as a vertical cavity surface emitting laser (“VCSEL”) emits light in a single direction through an upper surface of the laser structure. Many different optical packages can be used to house an optical transmitter. One common datacom-grade laser diode package is the so-called TO-can package. The TO-can package comes in a variety of sizes and configurations. The TO-can package can be further assembled with a variety of optical barrels, such as LC and SC barrels for aligning and coupling the optical transmitter with an optical fiber.
The TOSA may further include additional components. The TOSA can include a monitor, such as a photodiode, that generates feedback concerning performance parameters of the laser. In certain applications, the laser (e.g., a VCSEL) emission must be monitored to determine whether the power of the emitted light exceeds certain predetermined threshold magnitudes (i.e. a tolerance of a desired value). The monitoring function may be used to avoid any possible danger from the emission of unacceptably high power levels from the laser. The monitoring function may also be necessary in order to maintain a required level of average optical power necessary to obtain the desired optical modulation amplitude for a given on/off extinction ratio.
Semiconductor lasers are typically manufactured using methods of epitaxial growth upon a substrate. Many lasers can be manufactured at one time in the form of a wafer. Manufactured lasers, however, often differ in several different output parameters. In addition, for a laser to be operated at high speeds it is often required to operate the laser at levels that may produce optical output parameters that do not meet industrial specifications and requirements. Thus, compensation for different parameters of optical output of lasers must be conducted so that each device containing a laser that is sold satisfies various conditions.
One way that the various parameters of lasers are compensated for is attenuation. Attenuation of various parameters can be accomplished using various methods and apparatus known to one of ordinary skill. Selection and application of attenuation can be a critical process for successful production of VCSEL TOSAs meeting eye safe requirements for 10 Gbps Ethernet. Such a requirement can be set by industry standards, such as defined in the IEEE 802.3 ae standard. Two factors that may attribute to the need for attenuation are the need to bias a VCSEL at a substantial multiple of lasing threshold current to meet the dynamic performance requirements of operation (e.g. rise/fall times, ringing, eye mask, jitter), and tightly controlling the manufacturing distributions of the parts to meet all of the demanding requirements for an application. These requirements can become more stringent as the speed of a transmission system increases.
One method of attenuation is to use a window of an optical package to attenuate the optical signal transmitted from the VCSEL so that the optical signal satisfies several conditions. Because the conditions that must be satisfied are stricter at higher rates of data transmission, often there must be many more packages with different attenuation for these transmitters. For example, a system operating at 10 Gbps may require several times more optical packages with different attenuation to adjust output parameters of a VCSEL than that required for a system operating at 4 Gbps due to differences in the standards defined link budgets. Thus, manufacturers of high speed laser packages, assemblies, and devices have typically been required to keep many more packages on hand with many different attenuation characteristics to adjust optical output parameters of the lasers to meet industry conditions and requirements.
Therefore, for the reasons stated above, as well as others, an advantageous aspect of the present invention is improved methods and apparatuses for controlling output parameters of a TOSA.